Conventionally, with regard to an HST having a hydraulic pump and a hydraulic motor, there is well known an art in which a movable swash plate of the hydraulic pump or the hydraulic motor of variable capacity type is rotatively controlled by a hydraulic servo mechanism so as to adjust the capacity of the hydraulic pump or the hydraulic motor. As the hydraulic servo mechanism, there is known an automotive mechanism in which the movable swash plate is rotated automatically in proportion to the increase of rotation speed of the hydraulic pump by an electromagnetic valve attached to the hydraulic pump or the body of the HST and a manual servo mechanism in which the movable swash plate of the hydraulic pump is operated by a main shift lever attached to the outside of the body of the HST.
With regard to the manual servo mechanism, there is known construction in which the discharge amount of the hydraulic pump and the suction amount of the hydraulic motor are changed by rotating the main shift lever so as to perform the main speed change by changing the output rotation speed, and forward/rearward traveling is switched by operating the main shift lever oppositely from the neutral position. There is also known construction of control by rotating a forward/rearward running switch lever disposed near a steering wheel so as to improve operability in the case that the continuous forward/rearward running switching is required.
With regard to the control of the movable swash plate of the HST having such a hydraulic servo mechanism, for example, the control of the movable swash plate of the HST is disclosed in Patent Literature 1. In this control, when the forward/rearward running switch lever is operated to the rearward running side at the state in which the HST is shifted to the high speed position by operating the main shift lever, the movable swash plate of the hydraulic pump is rotated to the neutral position so as to prevent rearward start at high speed, thereby improving the safety.    Patent Literature 1: the Japanese Patent Laid Open Gazette 2002-250438